This invention relates to a method of cleaning heat exchangers wherein various deposits are removed therefrom.
This invention is particularly suitable for use in the situation in which residual heat in processed (stabilized) crude oil is transmitted to raw (unsatabilized) crude oil through shell and tube heat exchangers. In such a system, processed or stabilized crude oil from a fractionation tower is transmitted typically to the shell side of a series of shell and tube heat exchangers. Unstabilized crude, typically at a lower temperature than the stabilized crude, is charged to the tube side of the exchangers. The residual heat from the stabilized crude is accordingly transmitted to the unstabilized crude flowing through the tube side of the exchangers. Therefore, the unstabilized crude is preheated before being charged to the fractionation tower for processing. Before being charged to the fractionation apparatus for processing, the preheated unstabilized crude is further heated by external means, e.g., a furnace. During the operation of the heat exchangers, the exchangers usually begin to foul through the deposition of carbonaceous materials (usually paraffin waxes) and other solid deposits such as salt. These deposits cause a lowering of the heat transfer coefficient of the exchangers, thus causing the crude furnace to consume more fuel to bring the unstabilized crude to an adequate temperature for efficient fractionation. This represents a waste of energy which could otherwise be conserved.
Prior methods of removing the above mentioned deposits include reversing the direction of flow of shell and tube side fluids through the exchangers. This method removes or melts much of the deposits in the end exchangers in the series, but usually fails to melt deposits in the middle exchangers and may merely transfer the deposits to another area in the exchanger series. In the past, various equipment has also been used to warm up the exchangers to melt deposits therein. This technique however is very expensive due to the equipment utilized and requires removing exchangers from service. Various solvents have also been utilized to remove deposits. These solvents are typically introduced into one side of the exchangers. Usually, however, these solvents can be very expensive, and the equipment necessary for manipulating and recycling the solvents can also be very expensive and time consuming to operate. Reference is made at this point to U.S. Pat. No. 3,850,741 of Callahan et al which discloses one particular method utilizing solvents wherein the tube side fluid of a shell and tube heat exchanger is displaced by a particular solvent. The solvent is allowed to remain in the tube side of the exchanger for a period of about 8 to 24 hours to remove the deposits accordingly. During this period, it is necessary to take the exchangers out of service, thus contributing to the inefficiency of the entire preheating process.